feat: core tests for web search, schemas (#1339)

## Summary

Enhances web search tool functionality with comprehensive test coverage for various features including streaming, domain filtering, context sizes, multi-turn conversations, and max uses parameter.

## Changes

- Added five new comprehensive tests for web search tool capabilities:
  - Streaming web search responses
  - Domain filtering for search results
  - Different search context sizes (low/medium/high)
  - Multi-turn conversations with web search
  - Max uses parameter (Anthropic-specific)
- Fixed Anthropic provider to properly handle web search streaming events
- Added helper functions to validate web search sources and domain patterns
- Updated OpenAI test configuration to use gpt-4o instead of gpt-4o-mini
- Added JSON marshal/unmarshal tests for ResponsesToolMessageActionStruct and ResponsesTool

## Type of change

- [x] Feature
- [x] Refactor

## Affected areas

- [x] Core (Go)
- [x] Providers/Integrations

## How to test

Run the comprehensive tests for web search tool functionality:

```sh
# Run all web search tool tests
go test ./core/internal/testutil -run TestRunWebSearchTool

# Run specific web search tool tests
go test ./core/internal/testutil -run TestRunWebSearchToolStreamTest
go test ./core/internal/testutil -run TestRunWebSearchToolWithDomainsTest
go test ./core/internal/testutil -run TestRunWebSearchToolContextSizesTest
go test ./core/internal/testutil -run TestRunWebSearchToolMultiTurnTest
go test ./core/internal/testutil -run TestRunWebSearchToolMaxUsesTest
```

## Breaking changes

- [x] No

## Related issues

Improves test coverage for web search tool functionality.

## Security considerations

No security implications. Tests use standard web search functionality with public information.

## Checklist

- [x] I added/updated tests where appropriate
- [x] I verified builds succeed (Go)
- [x] I verified the CI pipeline passes locally

Author: akshaydeo

core/internal/testutil/tests.go

@@ -34,6 +34,11 @@ func RunAllComprehensiveTests(t *testing.T, client *bifrost.Bifrost, ctx context
 		RunEnd2EndToolCallingTest,
 		RunAutomaticFunctionCallingTest,
 		RunWebSearchToolTest,
+		RunWebSearchToolStreamTest,
+		RunWebSearchToolWithDomainsTest,
+		RunWebSearchToolContextSizesTest,
+		RunWebSearchToolMultiTurnTest,
+		RunWebSearchToolMaxUsesTest,
 		RunImageURLTest,
 		RunImageBase64Test,
 		RunMultipleImagesTest,